As a method of detecting the rotation speed or rotation angle of a DC motor with a brush taken to control the operation of the motor, there is conventionally known so-called sensorless positioning that utilizes a current ripple caused when contact between a brush and a commutator segment is switched. Generally, the motor current (armature current) of the DC motor with a brush rises at an inclination according to the inductance of a motor winding when the commutator segment contacting the brush is switched from one to another and then varies according to the difference between a power supply voltage and a counter-electromotive voltage. That is, the motor current is not constant but involves a pulsation (ripple) corresponding to a motor rotation position.
Such a current ripple is superimposed on a motor current. Thus, there have been proposed various methods of detecting the motor rotation speed, motor rotation direction, and the like without use of a rotation detection member such as a hall IC by extracting the current ripple from the motor current, followed by detection of a characteristic waveform from the extracted current ripple or by shaping of the extracted current ripple into a pulse waveform and detecting, e.g., a zero-cross point. For example, Patent Document 1 describes a configuration that detects a spiky pulse output generated in the motor current when one commutator segment contacting the brush is switched to next commutator segment to thereby detect the motor rotation speed. Further, Patent Document 2 describes a configuration that improves rotation detection accuracy by increasing the current ripple when detecting the motor rotation speed by detecting the current ripple contained in the motor current.